[css-color4] Defined handling of tagged images. Fix w3c#286

Author: svgeesus

css-color-4/Overview.bs

@@ -25,6 +25,17 @@ spec:css-color-4; type:descriptor; for:@color-profile; text:src
 spec:css-color-4; type:dfn; text:colorspace
 spec:css-color-4; type:dfn; text:gamut
 </pre>
+
+<pre class=biblio>
+	{
+		"TIFF": {
+			"href": "https://www.adobe.io/content/udp/en/open/standards/TIFF/_jcr_content/contentbody/download/file.res/TIFF6.pdf",
+			"title": "TIFF Revision 6.0",
+			"date": "3 June 1992"
+		}
+	}
+</pre>
+
 <link rel="stylesheet" href="style.css" />
 
 <style>
@@ -334,15 +345,48 @@ Accessibility and Conveying Information By Color</h3>
 	> Color is not used as the only visual means of conveying information,
 	> indicating an action, prompting a response, or distinguishing a visual element</a>
 
+<h3 id="tagged-images">Color Space of Tagged Images</h3>
+
+	An <dfn export>tagged image</dfn> is an image
+	that is explicitly assigned a color profile,
+	as defined by the image format.
+	This is usually done by including an
+	International Color Consortium (ICC) profile [[!ICC]].
+
+	For example JPEG [[JPEG]], PNG [[PNG]] and TIFF [[TIFF]]
+	all specify a means to embed an ICC profile.
+
+	Image formats may also use other, equivalent methods, often for brevity.
+
+	For example, PNG specifies a means to explicitly
+	tag an image as being in the sRGB colorspace,
+	without including the sRGB ICC profile.
+
+	Tagged RGB images,
+	and tagged images using a transformation of RGB such as YCbCr,
+	must be treated as being in the specified color space.
+
+	For example, when a browser running on a system with a Display P3 monitor
+	displays an JPEG image
+	tagged as being in the Adobe® RGB (1998) [[!AdobeRGB]]
+	colorspace, it must convert the colors
+	from Adobe® RGB (1998) to Display P3
+	so that they display correctly.
+	It must not treat the Adobe® RGB (1998) values
+	as if they were Display P3 values, which would produce incorrect colors.
+
 <h3 id='untagged'>
 Color Spaces of Untagged Colors</h3>
 
-	Colors specified in HTML and [=untagged images=] are in the sRGB color space ([[!SRGB]])
+	Colors specified in HTML,
+	and [=untagged images=] must be treated
+	as being in the sRGB color space ([[!SRGB]])
 	unless otherwise specified.
 
 	An <dfn export>untagged image</dfn> is an image that is not explicitly assigned a color profile, as defined by the image format.
 
-	Note that this rule does not apply to videos, since untagged video should be presumed to be in an ITU-defined colorspace.
+	Note that this rule does not apply to untagged videos, since
+	<dfn export>untagged video</dfn> should be presumed to be in an ITU-defined colorspace.
 	* At below 720p, it is Recommendation ITU-R BT.601 [[!ITU-R-BT.601]]
 	* At 720p, it is SMPTE ST 296 (same colorimetry as 709) [[!SMPTE296]]
 	* At 1080p, it is Recommendation ITU-R BT.709 [[!ITU-R-BT.709]]